The present invention is directed toward optical amplifiers having a substantially flat spectral gain.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. In a WDM system, plural optical signal channels are carried over a single optical fiber with each channel being assigned a particular wavelength. Such systems typically include a plurality of receivers, each detecting a respective channel by effectively filtering out the remaining channels.
Optical channels in a WDM system are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM optical signal channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50 km without significant attenuation. For distances beyond 50 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with Erbium. The Erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, Erbium doped fiber amplifiers do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm, for example, is typically amplified 4 dB more than an optical channel at a wavelength of 1555 nm.
While such a large variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated for a system with plural optical amplifiers or numerous, narrowly-spaced optical channels. In these environments, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Accordingly, optical amplifiers providing substantially uniform spectral gain have been developed. In particular, two stage optical amplifiers including an optical filter provided between first and second stages of Erbium doped fiber are known to provide gain flatness. In these amplifiers, the first stage is operated in a high gain mode and supplies a low noise figure to the second stage, while the second stage is operated in a high power mode. Although the second stage introduces more noise than the first, the overall noise output by the amplifier is low due to the low noise figure of the first stage. The optical filter selectively attenuates the high gain wavelengths, while passing the low gain wavelengths, so that the gain is substantially equal for wavelength output from the second stage.
Various improvements to such conventional amplifiers were patented by the assignee of the present invention. Particularly, U.S. Pat. Nos. 6,057,959; 5,963,361; 6,049,413; and 6,061,171 disclose and claim various gain-flattened optical amplifiers. These disclosed amplifiers utilize two-stage amplification in which two stages of Erbium-doped fiber are pumped and in which an inter-stage variable optical attenuator is controlled. Various types of variable optical attenuator control are disclosed including adjusting the attenuation according to the gain of the first and second stages or the ASE (amplified spontaneous emission) of the first and second stages.
Although these previously patented amplifiers provide excellent gain flatness and a low noise figure there is still room for improvement.
As noticed by the inventors, these two stage, gain-flattening amplifiers are typically designed to receive optical signals at a particular power level. Specifically, a flat gain response may be achieved when the average population inversion of Erbium ions is at a particular level. This corresponds to a fixed input power level (for a particular set of pump power levels). When the input power level varies from this optimal input power level, the gain flatness suffers.
In other words, when the total power level of all optical signals input to the amplifier differs from the desired input level, the amplifier can no longer amplify each wavelength with substantially the same amount of gain. Accordingly, the conventional gain-flattened amplifiers discussed above are unable to receive input optical signals over a wide range of power levels while maintaining substantially uniform gain at each wavelength.